Police are trying to find the suspects who shot 2 officers in Ferguson

A police officer pulls up tape marking the perimeter around Ferguson Police Department in Ferguson, Missouri, March 12, 2015. REUTERS/Jim Young FERGUSON, Mo. (Reuters) - The shooting of two police officers at a protest in Ferguson, Missouri, triggered a sweeping manhunt for suspects on Thursday and ratcheted up tensions in a city at the center of a national debate over race and policing.

U.S. President Barack Obama condemned the attack on the officers, who were released from a hospital after being treated for wounds, and Attorney General Eric Holder called it a "pure ambush."

"This was not someone who was trying to bring healing to Ferguson; this was a damn punk," Holder told reporters.

The pair were hit by gunfire outside police headquarters in the St. Louis suburb during a rally by protesters demanding sweeping changes after a scathing U.S. Justice Department report detailed deep-rooted racial bias within Ferguson's mostly white police force.

The demonstration was the latest of many held in the city since the killing in August of an unarmed black teenager by a white police officer, which also prompted protests around the country, and the federal investigation.

While condemning the wounding of the officers, organizers vowed more protests Thursday night, as well as a candle-light prayer vigil for peace which was attended by about 40 people a short distance up the road from the police station.

"We deplore all forms of violence," said Reverend Osagyefo Sekou, who was in the crowd when shots rang out. "But we also deplore the findings of the Department of Justice report and the suffering and the misery that this community has endured."

To prevent further bloodshed, officials said St. Louis County police and the Missouri Highway Patrol will take over security from the Ferguson force during any demonstrations. A member of a St. Louis County investigative team speaks with a resident living in the neighborhood near the Ferguson Police Department, in Ferguson, Missouri March 12, 2015. REUTERS/Kate Munsch NO ARRESTS MADE

Throughout the day, St. Louis County investigators canvassed streets near the police station, peering into trash cans, down drains, and quizzing residents about what they saw or heard.

Investigators brought in several people for questioning, but all of them were later released and no arrests were made, the St. Louis County Police said. Two Missouri congressman offered a $3,000 reward for information leading to the culprit.

The shooting left a 41-year-old St. Louis County police officer with a shoulder wound, and a 32-year-old officer from nearby Webster Groves Police Department with a bullet lodged near his ear, St. Louis County Police Chief Jon Belmar said.

Alongside the manhunt, the investigation has centered on exactly where the shots were fired from.

Belmar said the shooter had used a handgun and that shell casings were recovered, and he said the bullets came from the middle of the crowd of protesters.

"I don't know who did the shooting ... but somehow they were embedded in that group of folks," Belmar told a news conference.

Demonstrators at the scene insisted the shooter had been behind them, on a street leading away from the police station.

"The shooter was not with the protesters. The shooter was atop the hill," activist DeRay McKesson said on Twitter. A police officer walks out from the Ferguson Police Department in Ferguson, Missouri, March 12, 2015. REUTERS/Jim Young 'HEINOUS ACT'

In one video recorded at the chaotic scene after the gunfire, a witness can be heard calling out, "Acknowledgment nine months ago would have kept that from happening."

The shooting came less than three months after a man ambushed two New York City patrolmen, apparently seeking to avenge the killings of 18-year-old Michael Brown in Ferguson and an unarmed black man in New York.

Ferguson was rocked by two nights of rioting after a grand jury recommended no charges be filed in Brown's death.

His parents condemned Thursday's violence and said in a statement they are praying for the wounded officers.

"The heinous act of this individual does not reflect or forward the peaceful and non-violent movement that has emerged in our nation to confront police brutality," they said.

Wednesday night's rally came just hours after the resignation was announced of Ferguson Police Chief Tom Jackson, a long-held demand of the demonstrators.

He was the latest in a string of Ferguson officials to quit following the release of the Justice Department report, which found the city used police officers to issue traffic citations to black residents to boost its coffers. The harassment created a "toxic environment," the report said.